Expose control of the number of open files in libvips' cache.

Breaks API of existing cache method.
Disable libvips cache for I/O tests.

test/unit/colourspace.js

@@ -5,8 +5,6 @@ var assert = require('assert');
 var sharp = require('../../index');
 var fixtures = require('../fixtures');
 
-sharp.cache(0);
-
 describe('Colour space conversion', function() {
 
   it('To greyscale', function(done) {